Working arm for construction machine and method of producing the same

ABSTRACT

A wide plate-like material ( 27 ), having thin and thick wall portions in a transverse direction, is prepared by alternately joining one of flat thin plates ( 28, 29 ) and one of thick corner plates ( 30 ) or thick plates ( 31 ). The wide plate-like material ( 27 ) is bent at and along right and left thick corner plates ( 30 ) and formed into a U-shaped structure ( 32 ) which is U-shape in cross section. Separately from the U-shaped structure ( 32 ), a plate-like member ( 33 ) is prepared by butt welding thick plates ( 35 ) to right and left sides of a thin plate ( 34 ). The plate-like member ( 33 ) is welded to a lower side of the U-shaped structure ( 32 ) by laser welding or the like to form a square tubular structure, which is in a square shape in cross section and serves as a major part of an operating arm.

TECHNICAL FIELD

This invention relates to an operating arm for a construction machine,and more particularly to an operating arm suitable for use on aconstruction machine, for example, such as an excavating machine like ahydraulic excavator, and a method of fabrication of such operating arm.

BACKGROUND ART

Generally, a hydraulic excavator, typical of construction machines, islargely constituted by an automotive base structure, a revolvingstructure which is rotatably mounted on the base structure, and aworking mechanism as a front part liftably mounted on a front portion ofthe revolving structure, including a boom, an arm and a front attachment(e.g., a bucket).

An operating arm, such as the boom and arm, of the working mechanism(front part) is formed in a square tubular structure of a square shapein cross-section, for example, by joining together four steel plates,i.e., an upper plate, a lower plate, a right side plate and a left sideplate (e.g., as disclosed in Japanese Patent Laid-Open No. H11-21939).

For the purpose of enhancing rigidity and at the same time for reducingweight, operating arms on conventional construction machines of thissort are usually fabricated by the use of an upper plate which isprovided with a thin wall portion between right and left thick wallportions. Likewise, a lower plate is provided with a thin wall portionbetween right and left thick wall portions. A square tubular structureis formed by joining right and left side plates with the thick wallportions of the upper and lower plates by butt-welding. In this way,attempts have been made to guarantee high rigidity to a square tubularstructure despite reductions in weight.

Further, according to another prior art, a square tubular structure ofan operating arm for a construction machine is formed by a combinationof four corner members which are located at four corner portions(corners), and four flat plates joined between the corner members (e.g.,as disclosed in Japanese Patent Laid-Open No. 2001-20311).

In this case, in order to prevent concentration of stress in cornerportions of the operating arm formed as the square tubular structure,each one of the four corner members located at the corners is formed ina curved (or rounded) L-shape in cross-section beforehand. These cornermembers are joined with the flat plates afterwards by welding to formthe square tubular structure which is square in cross-section.

In this regard, in the case of the first-mentioned prior art, thick wallportions are provided in right and left side portions of upper and lowerplates, and right and left side plates are joined with the thick wallportions of the upper and lower plates by butt-welding as mentionedabove. Thus, in this case, since the upper and lower plates are notrequired to have a large thickness in their entire bodies, there is anadvantage that the weight of the operating arm can be reduced whileguaranteeing a certain degree of rigidity.

However, in the case of the prior art just mentioned, at the time ofjoining right and left side plates with right and left thick wallportions of the upper and lower plates by butt welding, the right andleft side plates have to be placed in a precisely aligned positionbetween the upper and lower plates by the use of an aligning jig of acomplicate shape. Further, in this case, there is a problem that the armhas to be assembled by a 3D welding operation which requires enormouslabor and time.

Following problems arise especially in case high energy density weldingsuch as laser welding is used for 3D welding. Namely, when joined by 3Dwelding, gaps are likely to occur between joining surfaces of the upperand lower plates and right and left side plates. If a gap of 0.5 mm orgreater exists between joining surfaces, for example, the vicinity ofthe joining surfaces may come out of a laser irradiation range, failingto form a joint of sufficient strength.

In addition, each one of corner portions in the above-mentioned squaretubular structure is constituted by a thick wall portion of the upper orlower plate and a joining portion (a welding portion) of the right orleft side plate. Therefore, the welding portion in the respective cornerportions are susceptible to residual stress or concentration of stress,and are difficult to ensure sufficient rigidity as an operating arm of aconstruction machine.

On the other hand, in the case of the second prior art mentioned above,corner portions of the operating arm formed as a square tubularstructure are formed by corner members of rounded L-shape incross-section, which has an advantage of suppressing influences ofresidual stress and concentration of stress.

However, in the case of this second prior art, the four corner membersas well as the four flat plates which interconnect the four cornermembers are formed of steel plates which are substantially uniform inthickness. Therefore, in this case, it is difficult to satisfy twocontradictory demands, i.e., weight reduction and high rigidity of anoperating arm. That is to say, there is a problem that the weight of theoperating arm as a whole is increased if thick steel plates are used toguarantee high rigidity.

If an operating arm is fabricated by the use of thinner steel plates forthe sake of weight reduction, corner members and flat plates have to bebutted against each other in a precisely aligned state at the time ofjoining them together, for example, by 3D welding which requires a greatdeal of labor and time for alignment of joining parts.

DISCLOSURE OF THE INVENTION

In view of the above-discussed problems with the prior art, it is anobject of the present invention to provide an operating arm for aconstruction machine, the arm being fabricated in a square tubularstructure by the use of a plural number of joined plates of differentthicknesses to achieve two contradictory aims, weight reduction andretention of high rigidity, and to provide a method of fabricating anoperating arm of the sort just mentioned.

It is another object of the present invention to provide an operatingarm for a construction machine, assembled with sufficiently strong jointstrength and efficiently by means of 2D welding which requires fareasier parts alignment as compared with 3D welding, and a method offabrication of the operating arm structure.

In accordance with the present invention, in order to achieve theabove-stated objectives, there is provided an operating arm for aconstruction machine for use as a front part of a construction machine,the operating arm being constituted by a plural number of joined platesand in the shape of a square tubular structure of a square shape incross section.

The operating arm according to the present invention is characterized inthat: the plural number of joined plates include flat thin plates to beformed into flat sections of the square tubular structure and thickcorner plates being greater in thickness in a flat shape than the flatthin plates joined side to side with the flat thin plates beforehand andbent into a convexly curved shape afterwards to form corner portions ofthe square tubular structure.

With the arrangements just described, by using steel plates of differentthicknesses as the flat thin plates and the thick corner plates, therecan be obtained a plate material of versatile utility and can be adoptedas a starting material in the fabrication of an operating arm. The thickcorner plates are in a flat shape before being bent into a convexlycurved shape in a bending stage. When in a flat shape, the thick cornerplates can be brought into a butt welding position simply by abuttingits joining side against a flat thin plate. For example, the flat thinplates and the thick corner plates can be joined together easily by 2Dwelding. Namely, by abortion of 3D welding as in the above-mentionedprior art, joining parts can be aligned and set in relative positions inan extremely facilitated manner. Besides, it becomes possible toincrease the thickness of thick corner plates which form corner portionsof the square tubular structure while reducing the thickness of flatthin plates which form flat side sections of the square tubularstructure, for providing an operating arm which is satisfactory inrigidity but reduced in weight as a whole.

Namely, as a result of an analysis of structural strength required ofoperating arms of construction machines, the inventors of the presentinvention have found that it is necessary for the square tubularstructure of an arm to have an ample wall thickness in corner portionsof the square tubular structure for the sake of rigidity, but a share ofload in flat side sections located between the corner portions is farsmaller as compared with the corner portions.

Therefore, the weight of the operating arm as a whole can be minimizedby reducing the thickness of flat thin plates which constitute flat sidesections of the square tubular structure. On the other hand, the thickcorner plates which constitute corner portions of the square tubularstructure are increased in thickness to guarantee enhanced rigidity ofthe operating arm as a whole. Accordingly, the square tubular structurewhich is constituted by the combination of flat thin plates and thickcorner plates has sufficient strength for supporting reaction forceswhich are imposed on the operating arm during an excavating operation orthe like, and provided sufficient rigidity as the operating arm.

Further, according to a preferred form of the present invention, thethick corner plates and the flat thin plates are joined together by sideto side butt welding to form a wide plate-like material havingalternately thick and thin wall portions in a transverse direction, thewide plate-like material being bent along the thick corner plates toform a U-shaped structure in cross section for use as a part of thesquare tubular structure.

In this case, the wide plate-like material which is prepared by buttwelding flat thin plates and thick corner plates is formed into U-shapein cross section by bending same at the positions of the thick cornerplates, for example, on a press to fabricate a U-shaped structure to beused as a major part of a square tubular structure which is in squareshape in cross section.

Further, according to a preferred form of the present invention, theflat thin plates and the thick corner plates are joined by side to sidebutt welding such that surfaces of the flat thin plates are positionedflush with the thick corner plates on one side in the direction ofthickness but indented from the thick corner plates on the other side inthe direction of thickness.

In this case, the one side in the direction of thickness is prepared forthe outer side of the square tubular structure, the square tubularstructure has smoothly joined surfaces on the outer side, instead of theother side where surfaces of the flat thin plates are indented from thethick plates due to a difference in plate thickness.

According to another preferred form of the present invention, the flatthin plates and the thick corner plates are joined together by side toside butt welding such that surfaces of the flat thin plates areindented from the thick corner plates on one side in the direction ofthickness but positioned flush with the thick corner plates on the otherside in the direction of thickness.

In this case, exertion of tensile stress at welded joint portionsbetween the flat thin plates and the thick corner plates can besuppressed to a low level at the time of bending the thick corner platesin such a direction as to expose raised and indented surfaces on theouter side of the square tubular structure, preventing development ofcracks from welded joint portions. In addition, the raised and indentedsurfaces of the thick and thin plates on the outer side of the squaretubular structure, so that one can utilize the raised and sunken surfacefor putting an emphasis on sturdiness in arm design for adding to acommercial value as an operating arm of a construction machine.

According to still another preferred form of the present invention, theflat thin plates and the thick corner plates are joined together by sideto side butt welding such that surfaces of the flat thin plates areindented from the thick corner plates on both sides in the direction ofthickness.

In this case, the raised and indented surfaces of the thick and thinplates can be exposed on the outer side of the square tubular structure,utilizing the design effects of the raised and indented surfaces foradding to a commercial value as an operating arm of a constructionmachine.

Further, according to the present invention, a boss mounting thick plateto be formed a boss mount member of the front part is joined with onelongitudinal end of the flat thin plates and thick corner plates of thesquare tubular structure prior to a bending operation, the boss mountingthick plate being bent into U-shape simultaneously with the thick cornerplates.

In this case, a boss mounting thick plate to be formed a boss mountmember of the front part is joined with one longitudinal end of flatthin plates and thick corner plates beforehand, and bent into U-shapesimultaneously with the thick corner plates, reducing steps of bendingoperations to make the fabrication process more efficient.

Further, according to the present invention, the boss mounting thickplate is substantially of the same thickness as the thick corner plates.Therefore, the boss mounting thick plate and the thick corner plates canbe bent simultaneously under uniform distribution of stress and loads.

On the other hand, according to the present invention, there is alsoprovided a method of fabricating an operating arm for a constructionmachine for use as a front part of a construction machine, the operatingarm being constituted by a plural number of joined plates and in theshape of a square tubular structure of a square shape in cross section,characterized in that the method comprises: a first welding stage forpreparing a wide plate-like material having alternately thick and thinwall portions in a transverse direction by butt welding side to side theplural number of joined plates in diffirent thicknesses to form thesquqre tubular structure; a bending stage for bending the wideplate-like material along thick plate portions to form corner portionsof the square tubular structure, and to form a U-shaped structure havinga U-shape in cross section through plastic deformation; a second weldingstage for welding a separate plate-like member to the U-shaped structureto close an opening of the latter to form the square tubular structureof a square shape in cross section.

By adoption of the method just described, a wide plate-like materialhaving thin and thick wall portions alternately in a transversedirection is prepared in a first welding stage by joining a pluralnumber of thin and thick joined plates by side to side butt welding,more particularly, by 2D butt welding. In a succeeding bending stage,the wide plate-like material is bent along thick wall portions to form aU-shaped structure which is U-shape in cross section. In a secondwelding stage, a separately prepared plate-like material is joined withthe U-shaped structure to close an opening in the U-shaped structure. Asa result, a square tubular structure having a square shape in crosssection is fabricated from the U-shaped structure for use as anoperating arm.

Further, according to the present invention, the first welding stagefurther comprises welding a boss mounting thick plate to be formed aboss mount member of the front part to one longitudinal end of the wideplate-like material, and the bending stage comprises bending the bossmounting thick plate into U-shape in cross section simultaneously whenthe wide plate-like material is bent to form the U-shaped structure.

In this case, a boss mounting thick plate to be formed a boss mountmember of the front part is joined with one longitudinal end of the wideplate-like material prior to the bending stage, and bent into U-shapetogether with the wide plate-like material in the bending stage,permitting to reduce the steps of bending operation and to enhance theefficiency of fabrication process. Further, in a case where a bossmounting thick plate is joined with one longitudinal end of a wideplate-like material, it becomes possible to enhance further the strengthof welded joints (between thin and thick plates) of the wide plate-likematerial prior to the bending stage, suppressing adverse effects ofbending loads which might otherwise posed on flat thin plate portions ofthe wide plate-like material.

Further, according to the present invention, the thin and thick platesare joined by high energy density welding of deep penetration in thefirst welding stage. In this case, it becomes possible to enhance thestrength of welded joints in the wide plate-like material which iscomposed of a plural number of thin and thick plates by adoption of highenergy density welding of deep penetration, securing sufficient jointstrength against loads which are imposed in the bending stage.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a front view of a hydraulic excavator adopting to a firstembodiment of the present invention;

FIG. 2 is an enlarged front view of the arm of FIG. 1, showing the armalone;

FIG. 3 is a plan view of a wide plate-like material to be used in thefabrication of an operating arm, and a boss mounting thick plate;

FIG. 4 is a perspective view of the wide plate-like material in FIG. 3,taken from the above slant direction;

FIG. 5 is an enlarged sectional view of the wide plate-like material,taken in the direction of arrows V-V in FIG. 3;

FIG. 6 is a sectional view of a U-shaped structure which is formed bybending the wide plate-like material of FIG. 5 into U-shape;

FIG. 7 is a sectional view of a plate-like member to be joined with theU-shaped structure;

FIG. 8 is a sectional view of the plate-like member and the U-shapedstructure which are joined together to form a square tubular structure;

FIG. 9 is an exploded perspective view of the plate-like member and theU-shaped structure which are to be joined together;

FIG. 10 is an exploded perspective view of the boss mounting thick platefor forming a boss mount member and a plate-like member to be joinedtogether;

FIG. 11 is a plan view of another boss mounting thick plate for forminga boss mount member, different from the boss mounting thick plate ofFIG. 10;

FIG. 12 is a perspective view of a boss mount member which is formed bybending the boss mounting thick plate of FIG. 11;

FIG. 13 is a plan view of a wide plate-like material and a boss mountingthick plate adopted in a second embodiment of the present invention;

FIG. 14 is an exploded perspective view of a U-shaped structure which isformed by bending the wide plate-like material of FIG. 13, and aplate-like member to be joined with the U-shaped structure;

FIG. 15 is a sectional view of a U-shaped structure and a plate-likemember adopted in a third embodiment of the present invention;

FIG. 16 is a sectional view of a square tubular structure which isformed by joining together the U-shaped structure and the plate-likemember of FIG. 15;

FIG. 17 is a sectional view of a square tubular structure which isformed by joining together a U-shaped structure and a plate-like memberadopted in a fourth embodiment of the present invention;

FIG. 18 is a perspective view of a wide plate-like material adopted in afifth embodiment of the present invention for forming a square tubularstructure;

FIG. 19 is a sectional view of the wide plate-like material, taken inthe direction of arrows XIX-XIX of FIG. 18;

FIG. 20 is a sectional view of a U-shaped structure formed by bendingthe wide plate-like material in U-shape, and a plate-like member to bejoined with the U-shaped structure;

FIG. 21 is a sectional view of a square tubular structure formed byjoining together the U-shaped structure and the plate-like member ofFIG. 20;

FIG. 22 is a sectional view of a square tubular structure which isformed by joining together a plate-like member and a U-shaped structureadopted in a sixth embodiment of the present invention;

FIG. 23 is a sectional view of a square tubular structure which isformed by joining together a plate-like member and a U-shaped structureadopted in a seventh embodiment of the present invention;

FIG. 24 is a perspective view of a wide plate-like material adopted inan eighth embodiment of the present invention for forming a squaretubular structure;

FIG. 25 is a sectional view of the wide plate-like material, taken inthe direction of arrows XXV-XXV of FIG. 24;

FIG. 26 is a sectional view of a U-shaped structure formed by bendingthe wide plate-like material of FIG. 25, and a plate-like member to bejoined with the U-shaped structure;

FIG. 27 is a fragmentary sectional view, showing part of the U-shapedstructure of FIG. 26 on an enlarged scale;

FIG. 28 is a sectional view of a square tubular structure which isformed by joining together the U-shaped structure and the plate-likemember of FIG. 26;

FIG. 29 is a sectional view of a square tubular structure which isformed by joining together a U-shaped structure and a plate-like memberadopted in ninth embodiment of the present invention;

FIG. 30 is a sectional view similar to FIG. 5, but showing a wideplate-like material before it is bent into the U-shaped structure shownin FIG. 29;

FIG. 31 is a sectional view of a square tubular structure which isformed by joining together a U-shaped structure and a plate-like memberadopted in a tenth embodiment of the present invention;

FIG. 32 is a sectional view similar to FIG. 5, but showing a wideplate-like material before it is bent into the U-shaped structure shownin FIG. 31;

FIG. 33 is a sectional view of a U-shaped structure which is formed bybending the wide plate-like material of FIG. 32, and a plate-like memberto be joined with the U-shaped structure;

FIG. 34 is a fragmentary sectional view, showing part of the U-shapedstructure of FIG. 33 on an enlarged scale; and

FIG. 35 is a front view of a hydraulic excavator in a modificationaccording to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereafter, with reference to the accompanying drawings, the operatingarm for a construction machine and a method of fabricating the operatingarm, according to the present invention, is described more particularlyby way of its preferred embodiments which are applied by way of exampleto a hydraulic excavator employing an offset type boom for its workingmechanism.

Of the accompanying drawings, shown in FIGS. 1 through 12 is a firstembodiment of the present invention. In these figures, indicated at 1 isa hydraulic excavator as a typical example of construction machines. Thehydraulic excavator 1 is largely constituted by an automotive crawlertype base structure 2, a revolving structure 3 which is rotatablymounted on the automotive base structure 2 and a working mechanism 11,which will be described hereinafter.

In the case of the particular embodiment shown, the revolving structure3 is largely constituted by a revolving frame 4, a cab 5 which isprovided on the revolving frame 4, an exterior cover 6, and acounterweight 7. The cab 5 is an operational housing which internallydefines an operating room to be occupied by an operator at the controlof the machine. Together with the counterweight 7, the exterior cover 6defines a machine room for accommodating an engine and a hydraulic pump(both not shown).

Indicated at 8 is a soil sweeper blade which is provided on the frontside of the automotive base structure 2. The soil sweeper blade 8 isliftable up and down relative to the base structurer 2, and used, forexample, for leveling a ground surface or for removing soil.

Denoted at 11 is an offset boom type working mechanism as a front partwhich is liftably provided in a front side of the revolving structure 3.This working mechanism 11 is constituted by a lower boom 12 which isliftably mounted on the revolving frame 4, an upper boom 13 which ispivotally attached to the fore end of the lower boom 12 for swingingmovements in rightward and leftward directions, an arm stay 14 which ispivotally attached to the fore end of the upper boom 13 for swingingmovements in rightward and leftward directions, an arm 21 which ispivotally attached to the fore end of the arm stay 14 for upward anddownward rotational movements, which will be described hereinafter, anda bucket 15 which is pivotally supported at the fore end of the arm 21as a front attachment.

In this instance, the lower boom 12, upper boom 13 and arm 21 of theworking mechanism 11 constitute an operating arm of the constructionmachine. In the case of the offset boom type working mechanism 11, alink rod (not shown) is pivotally connected between a fore end of thelower boom 12 and the arm stay 14 for swing movements in rightward andleftward directions.

The above-mentioned link rod form a parallel link mechanism togetherwith the lower boom 12, upper boom 13 and arm stay 14 thereby to keepthe arm 21 (the arm stay 14) constantly in parallel relation with thelower boom 12.

Further, a boom cylinder 16 is provided between the revolving frame 4and the lower boom 12, and an arm cylinder 17 is provided between thearm stay 14 and the arm 21. A bucket cylinder for the front attachmentis provided between the arm 21 and the bucket 15 and through links 18and 19.

Furthermore, an offset cylinder (not shown) is provided between thelower boom 12 and upper boom 13. The offset cylinder is expanded orcontracted, for example, at the time of a side ditch or side-gutterexcavating operation to move the arm 21 to the right or to the left inparallel relation with the lower boom 12 through the above-mentionedparallel link.

Indicated at 21 is an arm employed as an operating arm of the workingmechanism 11 for a construction machine. As shown in FIGS. 2 through 12,this arm 21 is constituted by a square tubular structure 22 extending inthe longitudinal direction, a boss mount member 23 joined with a coupleof boss portions 23A and 23B and located at one longitudinal end of thesquare tubular structure 22, a second boss mount member 24 joined with asingle boss portion 24A and located at the other longitudinal end of thesquare tubular structure 22, and a cylinder bracket 26, which will bedescribed hereinafter.

In this instance, as shown in FIG. 8, the square tubular structure 22which constitute a major part of the arm 21 is formed as a hollow tubewhich is substantially of a square shape in cross-section. Namely, thesquare tubular structure 22 is composed of a pair of upper cornerportions 22A which are located at right and left upper corners of thesquare tubular structure, an upper flat section 22B which is locatedbetween the upper corner portions 22A, a pair of lower corner portions22C which are located at the right and left lower corners of the squaretubular structure, a lower flat section 22D which is located between theright and left lower corner portions 22C, and right and left flatsections 22E located between the upper and lower corner portions 22A and22C.

The upper corner portions 22A of the square tubular structure 22 areeach formed of a thick corner plate 30, while the upper flat section 22Bis formed of flat thin plate 28, as described in greater detailhereinafter. Likewise, the lower corner portions 22C are each formed ofa thick plate 31, while the lower flat section 22D is formed of a thinplate 34, as described in greater detail hereinafter. The right and leftflat sections 22E are each formed of a flat thin plate 29 which will bedescribed hereinafter.

At the boss mount member 23 at one end of the arm 21, a link 18 which isshown in FIG. 1 is pivotally connected to the boss portion 23A through apin, and the bucket 15 is pivotally supported at the boss portion 23Bthrough a pin. On the other hand, at the boss mount member 24 at theother end of the arm 21, the arm stay 14 which is shown in FIG. 1 ispivotally connected to the boss portion 24A through a pin.

Indicated at 25 is a lid plate which closes the other end of the squaretubular structure 22 together with the boss mount member 24, and at 26is a cylinder bracket which is attached to the other end of the squaretubular structure 22 through the lid plate 25. In this instance, asshown in FIGS. 1 and 2, the cylinder bracket 26 is formed as a bracketplate substantially in the shape of a diverging fan and bored with acouple of pin-receiving holes 26A and 26B.

A rod side end of the arm cylinder 17, shown in FIG. 1, is pivotallyconnected through a pin at the pin-receiving hole 26A of the cylinderbracket 26, and a bottom side end of the bucket cylinder 20 is pivotallyconnected through a pin at the pin-receiving hole 26B.

Indicated at 27 is a wide plate-like material employed as a base orstarting material in the fabrication of the square tubular structure 22.As shown in FIGS. 3 to 5, this wide plate-like material 27 is composedof a longitudinally extending flat thin plates 28, 29, thick cornerplates 30 and thick plates 31, which are joined together side to side bybutt welding. More particularly, for example, these plate materials arepre-joined by high energy density welding like laser welding capable ofdeep penetration.

In this instance, the flat thin plate 28 which is located centrally ofthe wide plate-like material 27 is formed by the use of an elongatedlongitudinally extending flat steel plate. Similar to the flat thinplate 28, the right and left thick corner plates 30 which are joinedwith the opposite sides (right and left directions) of the flat thinplate 28 are each formed by the use of an elongated longitudinallyextending flat steel plate.

As shown in FIGS. 4 and 5, the flat thin plates 28 and 29 are joinedwith the thick corner plates 30 and the thick plate 31 by side to sidebutt welding in such a way as to be disposed substantially flush witheach other on one side (upper surface) in the direction of thickness,and raised and indented surface with each other on the other side (lowersurface) in the direction of thickness.

The thick corner plates 30 of the wide plate-like material 27 is greaterin thickness than the flat thin plate 28, and are bent in a convexlycurved shape along a center folding line 30A indicated by a broken linein FIGS. 3 and 4. By a bending operation, the thick corner plates 30 arebent into a curved (or rounded) L-shape in cross section as shown inFIG. 6 to form upper corner portions 22A of the square tubular structure22 as shown in FIG. 8.

Further, as shown in FIGS. 3 and 4, the right and left flat thin plates29 which are joined on the outer side of the right and left thick cornerplates 30 are each constituted by a thin wall steel plate substantiallyof a trapezoidal shape extending longitudinally along the thick cornerplate 30. The right and left thick plates 31 which are joined on theouter sides of the flat thin plate 29 are each constituted by a thicksteel plate extending longitudinally along and on the outer side of theflat thin plate 29.

In the case of the particular example shown, the flat thin plates 28 and29 are each formed of a steel plate which is, for example, 3 mm to 6 mmthick, preferably, 3.2 mm thick. On the other hand, the thick cornerplates 30 and the thick plates 31 is formed of a steel plate of a doublethickness (e.g., of 6 mm to 12 mm in thickness) as compared with theflat thin plates 28 and 29.

The wide plate-like material 27 which is composed of the thin plates 28and 29 and the thick plates 30 and 31 is joined with the boss mountmember 23 (of a boss mounting thick plate 37 which will be describedhereinafter) at a joining end 27A at one longitudinal end as shown inFIG. 3, and joined with the lid plate 25 at a joining end 27B at theother longitudinal end as shown in FIG. 2.

At the other longitudinal end, the wide plate-like material 27 isprovided with obliquely cut portions 27C extending obliquely outwardfrom opposite sides of the joining end 27B toward the outer side of thethick plates 31 and cut across the end portion of the flat thin plates29. The boss mount member 24 which is shown in FIGS. 2 and 12 is weldedto these obliquely cut portions 27C by means of high energy densitywelding or the like.

Denoted at 32 is a U-shaped structure which is formed by bending thewide plate-like material 27. More particularly, the U-shaped structure32 is formed by bending the thick plates 30 of the wide plate-likematerial 27 into a convexly curved shape along folding lines 30Aindicated by broken lines in FIG. 3. As a result of plastic deformationof the thick wall plates, the wide plate-like material is folded into aU-shape in cross section as shown in FIG. 6 by a bending operation.

In the course of the bending operation, the right and left thick cornerplates 30 bent into L-shape in cross section as shown in FIG. 6 to formcorner portions 22A of the square tubular structure 22 as shown in FIG.8. Further, the centrally located flat thin plate 28 defines an upperflat section 22B of the square tubular structure 22.

The right and left flat thin plates 29 form the right and left flatsections 22E of the square tubular structure 22. As shown in FIG. 7, anopening 32A is formed on the lower side of the U-shaped structure 32between the right and left thick plates 31. This opening 32A is closedby a plate-like member 33 which will be described hereinafter.

Indicated at 33 is a plate-like member which constitutes the squaretubular structure 22 together with the U-shaped structure 32. As shownin FIGS. 7 to 9, this plate-like member 33 is constituted by a centrallylocated thin plate 34, and right and left thick plates 35 which arejoined side to side with the thin plate 34 by high energy densitywelding.

In this instance, as shown in FIG. 9, the plate-like member 33 is formedin a length which corresponds to the length of the thick plates 31 ofthe U-shaped structure 32, and, as shown in FIG. 7, in a width whichcorresponds to the width between the right and left thick plates 31. Asshown in FIG. 7, the plate-like member 33 is fitted in the opening 32A(between the right and left thick plates 31) of the U-shaped structure32, and, at joint portions 36 at the opposite sides, securely joinedwith the thick plates 31 by means of high energy density welding or thelike.

As a consequence, the opening 32A of the U-shaped structure 32 is closedwith the plate-like member 33 from beneath to form the square tubularstructure 22 with a square cross-sectional shape as shown in FIG. 8.Right and left lower corner portion 22C are formed by the thick plates31 of the U-shaped structure 32 and the thick plates 35 of theplate-like member 33 in the vicinity of the above-mentioned jointportions 36. A flat section 22D at the bottom of the square tubularstructure 22 is defined by a lower surface of the plate-like member 33.

The thin wall plate 34 of the plate-like member 33 is formedsubstantially in the same thickness as the flat thin plates 28 and 29 ofthe wide plate-like material 27, while the thick plates 35 are formedsubstantially in the same thickness as the thick plates 30 and 31 of thewide plate-like material 27.

Indicated at 37 is a boss mounting thick plate to be formed into theboss mount member 23. This boss mounting thick plate 37 is formed in ashape as shown in FIG. 3, and in a thickness which is substantially sameas the thick corner plates 30 and the thick plates 31 of the wideplate-like material 27. The boss mounting thick plate 37 is providedwith a couple of boss mount holes 37A in which a tubular boss portion23A fixedly set by welding as shown in FIG. 2, and a couple ofsemi-circular grooves 37B in which another tubular boss portion 23B isfixedly set by welding as shown in FIG. 2.

As shown in FIG. 10, the boss mounting thick plate 37 is bent alongfolding lines 37C indicated by broken lines in FIG. 3. This bossmounting thick plate is formed into U-shape in cross section, which issubstantially similar to the above-described U-shaped structure 32.

Indicated at 38 is a plate-like member which makes up the boss mountmember 23 together with the boss mounting thick plate member 37.Substantially in the same manner as the plate-like member 33 of thesquare tubular structure 22, the plate-like member 38 is composed of acentrally located thin plate 38A and right and left thick plates 38B asshown in FIG. 10. However, in correspondence to the boss mounting thickplate 37, the plate-like member 38 is formed in a short length, andjoined with the boss mounting thick plate 37 being closed the lower sideopening of the boss mounting thick plate 37.

Thus, by joining together the boss mounting thick plate 37 and theplate-like member 38, the boss mount member 23 is formed in a shortsquare tubular structure which is square shape in cross section.Afterwards, the boss mount member 23 is joined with one longitudinal endof the square tubular structure 22, at a joining end 27A indicated inFIG. 2.

Designated at 39 is another boss mounting thick plate to be formed intothe boss mount member 24. This boss mounting thick plate 39 is formed inthe shape as shown in FIG. 11, and in a thickness which is same as thatof the thick plates 30 and 31 of the wide plate-like material 27.Further, the boss mounting thick plate 39 is provided with a couple ofsemi-circular grooves 39A in which a tubular boss portion 24A is to befixed by welding as shown in FIG. 2.

In this instance, along folding lines 39B indicated by broken lines inFIG. 11, opposite side portions of the boss mounting thick plate 39 arebent upward as shown in FIG. 12, and formed into the boss mount member24 of U-shape in cross section. This boss mount member 24 is joined withthe other longitudinal end of the square tubular structure 22 at theobliquely cut portions 27C shown in FIG. 2.

Being arranged as described above, the operating arm 21 of the hydraulicexcavator 1 according to the present embodiment is fabricated by amethod as follows.

In the first place, in a process of fabricating the square tubularstructure 22 which constitutes a major part of the arm 21, as shown inFIGS. 3 and 4, the centrally located flat thin plate 28, the thickcorner plates 30, the flat thin plates 29 and the outermost thick plates31 are successively joined side to side butt welding, for example, bythe use of a laser beam, to prepare the wide plate-like material 27 withalternately thin and thick wall portions in the transverse direction(First Welding Stage).

In the next place, the wide plate-like material 27 is bent on a pressmachine by the use of a die (not shown) to produce through plasticdeformation a U-shaped structure 32 which is U-shaped in cross sectionas shown in FIGS. 6 and 9 (Bending Stage). At this time of forming theU-shaped structure 32 on a press, the right and left thick corner plates30 of the wide plate-like material 27 are bent into a curved L-shape asshown in FIG. 6.

Further, separately from the U-shaped structure 32, the plate-likemember 33 is prepared by joining thick plates 35 with the opposite rightand left sides of the thin plate 34 by side to side butt welding asshown in FIG. 7. Then, the plate-like member 33 is joined with the sideof opening 32A of the U-shaped structure 32 by laser welding in such away as to close the opening 32A on the lower side of the U-shapedstructure 32 (Second Welding Stage).

As a result of the foregoing operations, the square tubular structure 22with a square cross-sectional shape as shown in FIG. 8 is produced fromthe U-shaped structure 32 and the plate-like member 33. Upper cornerportions 22A on the upper side of the square tubular structure 22 areformed by the thick corner plates 30, and the upper flat section 22B isformed by the flat thin plate 28.

Further, lower corner portions 22C on the lower side of the squaretubular structure 22 are formed at the joint portions 36 of the thickplates 31 and 35, and the lower flat section 22D is formed by the lowerside of the plate-like member 33 (the thin plate 34). The flat sections22E at the right and left lateral sides of the square tubular structure22 are formed by the flat thin plates 29 between the thick plates 30 and31.

In a process of fabricating the boss mount member 23, as shown in FIG.3, by the use of a press means, for example, a couple of circular bossmount holes 37A and a couple of semi-circular grooves 37B are firstlybored in the boss mounting thick plate 37, a starting material forforming the boss mount member 23.

Thereafter, on a press machine, the boss mounting thick plate 37 is bentalong the folding line 37C indicated by broken lines in FIG. 3, therebyshaping the boss mounting thick plate 37 into U-shape in cross sectionas shown in FIG. 10.

Separately from the boss mounting thick plate 37, the plate-like member38 is prepared by joining thick plates 38B with the opposite sides ofthe thin plate 38A by end to end butt welding as shown in FIG. 10. Then,the plate-like member 38 is joined with the boss mounting thick plate 37by laser welding in such a way as to close the opening on the lower sideof the boss mounting thick plate 37.

As a result of the foregoing operations, the boss mount member 23 of ashort square tubular form and of a square shape in cross section isformed from the boss mounting thick plate 37 and the plate-like member38. The formed boss mount member 23 is joined with one longitudinal endof the square tubular structure 22 by laser welding at the position of ajoining end 27A as shown in FIG. 2.

On the other hand, in a process of fabricating the boss mount member 24,as shown in FIG. 11, by the use of a press means, for example, a coupleof semi-circular grooves 39A are firstly bored in the boss mountingthick plate 39, a starting material for forming the boss mount member24.

Thereafter, the boss mounting thick plate 39 is bent on a press alongfolding lines 39B indicated by broken lines in FIG. 11. As aconsequence, the boss mounting thick plate 39 is formed into U-shape incross section as shown in FIG. 12. In the next place, the boss mountmember 24 is joined with the other longitudinal end of the squaretubular structure 22 by laser welding at the position of the obliquelycut portions 27C as shown in FIG. 2.

Further, as shown in FIG. 2, a lid plate 25 is joined with the otherlongitudinal end of the square tubular structure 22 by laser welding atthe position of a joining end 27B. As a result, the other end portion ofthe square tubular structure 22 is closed with the lid plate 25.

A cylinder bracket 26 is welded to the outer side of the lid plate 25 insuch a way as to extend toward the top side of the other end of thesquare tubular structure 22. Thus, the arm 21 which is intended for useof an operating arm of a construction machine is fabricated as shown inFIG. 2.

Similar to the arm 21, the above-described square tubular structure canbe applied to other operating arms of a working mechanism, for example,to the lower boom 12 and the upper boom 13 of the working mechanism 11shown in FIG. 1.

In the next place, the hydraulic excavator 1 with the offset boom typeworking mechanism 11 can be put in travel in the forward or reversedirection by driving the automotive base structure 2. The direction ofthe working mechanism 11 can be changed suitably by rotationally drivingthe revolving structure 3 on the automotive base structure 2.

At the time of a ground excavating operation, for example, the boomcylinder 16, arm cylinder 17 and bucket cylinder 20 are expanded orcontracted thereby operating the lower boom 12, arm 21 and bucket 15 ofthe working mechanism 11 to carry out an excavating operation.

In the case of the offset boom type working mechanism 11, the upper boom13 can be turned to the right or to the left of the lower boom 12 byexpanding or contacting an offset cylinder (not shown). Accordingly, aside ditch or gutter can be dug easily by shifting the position of thearm 21 to the right or to the left of the lower boom 12.

Further, when the lower boom 12 of the working mechanism 11 is turnedlargely in the upward direction with the arm 21 and the bucket 15 foldedinward toward the lower boom 12 as shown in FIG. 1, the workingmechanism 11 as a whole can be retained within a turn radius of therevolving structure 3, permitting to carry out a digging operationsmoothly without colliding against ambient obstacles even on a narrowworking site.

Thus, according to the present embodiment, fabrication of the squaretubular structure 22 which constitutes a major part of the arm 21 startsfrom the wide plate-like material 27 which has alternately thin andthick wall portions in the transverse direction and is prepared byjoining thick corner plates 30 with the opposite right and left sides ofthe centrally located flat thin plate 28, and then joining the flat thinplates 29 and thick plates 31 successively side to side on the outersides of the thick corner plates by laser welding as shown in FIGS. 3and 4. Next, the wide plate-like material 27 is bent in L-shape at eachone of the right and left thick corner plates 30, namely bent intoU-shape as a whole to obtain a U-shaped structure 32 which is formed inU-shape in cross section as shown in FIGS. 6 and 9.

Separately from the U-shaped structure 32, the plate-like member 33 isprepared by joining the thick plates 35 with the opposite right and leftsides of the thin plate 34 as shown in FIG. 7 by side to side buttwelding. Next, the plate-like member 33 is joined with the side of theopening 32A of the U-shaped structure 32 by laser welding in such a wayas to close the opening 32A on the lower side of the U-shaped structure32 with the plate-like member 33, forming the square tubular structure22 which is of a square shape in cross section as shown in FIG. 8.

As a consequence, there is obtained the square tubular structure 22, amajor part of the arm 21, having upper corner portions 22A formed by thethick corner plates 30 and having a upper flat section 22B formed by theflat thin plate 28. Further, lower corner portions 22C of the squaretubular structure 22 are formed in the vicinity of joint portions 36between the thick plates 31 and 35, and a lower flat section 22D isformed by the lower side of the plate-like member 33 (the thin plate34). Further, flat sections 22E at the opposite right and left lateralsides of the square tubular structure 22 are formed by the flat thinplates 29 between the thick plates 30 and 31.

According structural analysis conducted by the present inventors withregard to the operating arm (e.g., the arm 21), it has been revealedthat the square tubular structure 22 should have a large wall thicknessat the corner portions 22A and 22C to guarantee sufficient rigidity.However, the flat sections 22B, 22D and 22E between the corner portions22A and 22C are in positions to take a smaller part in sharing loads ascompared with the respective corner portions 22A and 22C. Namely, it hasbeen found that the flat sections 22B, 22D and 22E are not necessarilyrequired to be formed of a thick wall plate.

Therefore, in the present embodiment, the flat sections 22B, 22D and 22Eof the square tubular structure 22 are formed by the use of thin plates28, 29 and 34 for the purpose of reducing the total weight of the arm21. Further, the corner portions 22A and 22C of the square tubularstructure 22 are formed by the use of the thick corner plates 30 and thethick plates 31 and 35.

The above arrangements contribute to enhance the rigidity of the arm 21as a whole. That is to say, the arm 21 has sufficient strength forsustaining digging reaction forces which are exerted from the side ofthe bucket 15 during a digging operation. In addition, since the squaretubular structure 22 of the arm 21 is formed by the use of the steelplates which have alternatoly thin and thick wall portions, such as thethin plates 28, 29 and 34 in combination with the thick plates 30, 31and 35, versatile plate materials can be employed for the fabrication ofthe arm 21.

Furthermore, the wide plate-like material 27, a preparatory material forfabrication of the square tubular structure 22, can be formed by buttwelding alternately the thin plate 28 or 29 and the thick plate 30 or 31prior to the bending stage forming into a U-shaped structure 32. Thesethin and thick plates can be welded together by a 2D welding operation.

In this regard, for example, the flat thin plate 28, right and leftthick corner plates 30, right and left flat thin plates 29 and right andleft thick plates 31, which are shown in FIG. 3, are laid out on asurface table. The respective plates are laid face down, namely, in areversed state to lay on its outer or upper side thereof (the side whichis shown on the upper side in FIG. 5).

By laying out the respective plates 28, 30, 29 and 31 on the same planeon a surface table as described above, they can be joined easily side toside in a facilitated manner by butt welding, that is to say, by 2Dwelding. Adoption of 2D welding makes positioning and alignment ofjoining parts far easier as compared with 3D welding adopted by theprior art mentioned hereinbefore. Besides, thanks to 2D welding, it ispossible to carry out welding operations efficiently, forming welds ofsufficient strength.

Further, by using high energy density welding like laser welding fordeep penetration, the wide plate-like material 27 can be assembled withenhanced joint strength at the respective welded joints between the thinplate 28 or 29 and the thick plate 30 or 31. For example, it is possibleto form a complete weld getting to back side of a material from frontside.

High energy density welding like laser welding can improve fatigue lifeof welds as compared with partial penetration by arc welding or completepenetration by the use of a backing strip. In addition, high speedwelding, approximately five times as high as arc welding, is possible,with suppressed input heat. As a consequence, high energy densitywelding can reduce occurrence of post-welding deformations, especiallyto plates which are smaller than 10 mm like the thin plates 28 and 29.In addition, the respective plates can be joined with sufficient jointstrength against tensile loads which would be exerted in the bendingstage.

Moreover, at the time of bending the wide plate-like material 27 intothe U-shaped structure 32 as shown in FIG. 6, the side with indentedsurface portions, which are formed as a result of a difference inthickness between the flat thin plates 28 and 29 and the thick cornerplates 30, is disposed on the inner side, without being exposed on theouter side of the U-shaped structure 32. Accordingly, on the outer side,the thin plates are joined substantially flush with outer surfaces ofthe thick plates, without forming indented portions on the outer side ofthe U-shaped structure 32, namely, on the outer side of the squaretubular structure 22.

Further, as shown in FIGS. 7 and 9, at the time of welding theplate-like member 33 in the opening 32A of the U-shaped structure 32 bylaser welding to form the square tubular structure 22 as shown in FIG.8, the plate-like member 33 can be easily set in position simply byplacing to the U-shaped structure 32 which is formed on a press having acorner angle of approximately 90° in such a way as to close the opening32A.

Therefore, the U-shaped structure 32 and the plate-like member 33 can beeasily set in position relative to each other also in the longitudinaldirection of the U-shaped structure 32 as shown in FIG. 9. Thiscontributes to improve the efficiency of welding operations to a markeddegree. In addition, welds of sufficient strength can be formed thanksto complete welding by high energy density welding.

Thus, according to the present embodiment, the U-shaped structure 32 andthe wide plate-like material 27 is formed by the use of plates ofdifferent thicknesses, i.e., the flat thin plates 28 and 29 and thethick corner plates 30 and the thick plates 31, and the square tubularstructure 22 of a square cross-sectional shape is formed simply byjoining the plate-like member 33 in the opening 32A of the U-shapedstructure 32. As a result, it becomes possible to reduce the weight ofthe arm 21 as an operating arm, while securing sufficient rigidity ofthe arm.

In addition, the wide plate-like material 27 can be formed by joiningthe flat thin plates 28 and 29 with the thick corner plates 30 and thickplates 31 by 2D welding which is far simpler in positioning and aligningwelding parts as compared with 3D welding. This means that welds ofsufficient strength can be formed in an efficient manner.

Now, turning to FIGS. 13 and 14, there is shown a second embodiment ofthe present invention. In the following description of the secondembodiment, those component parts which are identical with thecounterparts in the foregoing first embodiment are simply designated bythe same reference numerals or characters to avoid repetitions of sameexplanations.

In short, a feature of this embodiment resides in that boss mountingthick plates 41 and 42 are joined with longitudinal ends of a wideplate-like material 27, which is composed of flat thin plates 28 and 29and thick corner plate 30 and thick plates 31 (at a joining end 27A andobliquely cut portions 27C), and then the boss mounting thick plates 41are bent together with the wide plate-like material 27.

In this instance, a couple of circular boss mount holes 41A and a coupleof semi-circular boss mount grooves 41B are bored in the boss mountingthick plate 41 substantially in the same manner as the boss mountingthick plate 37 in the foregoing first embodiment. Along folding lines41C indicated by broken lines in FIG. 13, the boss mounting thick plate41 is bent to form a boss mount member 23 as shown in FIG. 2.

However, in the case of the boss mounting thick plate 41, it is joinedwith the joining end 27A of the wide plate-like material 27 by the useof the high energy density welding like laser welding before prior to abending stage, and then bent together with the wide plate-like material27 as shown in FIG. 14 to obtain a U-shaped structure 43, which will bedescribed hereinafter.

Besides, in order to form boss mount member 24 as exemplified in FIG. 2,the other boss mounting thick plates 42 are adopted in the presentembodiment in place of the boss mounting thick plates 39 in theforegoing first embodiment. Each one of the boss mounting thick plates42 is formed substantially in a triangular shape as shown in FIG. 13,and substantially in the same thickness as the thick plates 30 and 31 ofthe wide plate-like material 27.

Further, the boss mounting thick plates 42 are each provided with asemi-circular boss mount groove 42A in which a boss portion 24A asexemplified in FIG. 2 is fixedly anchored by welding. These bossmounting thick plates 42 are joined with the other longitudinal end ofthe wide plate-like material 27 by laser welding or the like atobliquely cut portions 27C shown in FIG. 13.

Indicated at 43 is a U-shaped structure which is formed by bending ajoined assembly of the wide plate-like material 27 and the boss mountingthick plates 41 and 42. This U-shaped structure 43 is shapedsubstantially in the same manner as the U-shaped structure 32 in theforegoing first embodiment, and joined with a plate-like member 44 lateron to form a square tubular structure 22, which constitutes a major partof the arm 21.

However, in the case of the present embodiment, the U-shaped structure43 is formed by joining the boss mounting thick plates 41 and 42 withthe wide plate-like material 27 before pressing same into U-shape incross section as shown in FIG. 14. Thus, in this case, the boss mountingthick plates 41 and 42 constitute part of the U-shaped structure 43.

Indicated at 44 is a plate-like member which is adopted in the presentembodiment. This plate-like member 44 is formed substantially in thesame manner as the plate-like member 33 in the foregoing firstembodiment, and constituted by a centrally located thin plate 45, andright and left thick plates 46 which are joined with right and leftlateral sides of the thin plate 45 by laser welding or the like.

In this instance, as shown in FIG. 14, the plate-like member 44 isformed in a length which approximately corresponds to the lengths of thethick plates 31, 41 and 42 of the U-shaped structure 43, and in a widthwhich corresponds to the width of the spacing between the right and leftthick plates 31. Further, the plate-like member 44 is fitted in theopening on the lower side of the U-shaped structure 43 (between thethick plates 31) and anchored between the thick plates 31 by laserwelding or the like.

As a result, the opening on the lower side of the U-shaped structure 43is closed with the plate-like member 44 to form a square tubularstructure of a square shape in cross section similarly to the squaretubular structure 22 in the above-described first embodiment.

Thus, substantially in the same manner as in the foregoing firstembodiment, the wide plate-like material 27 and the U-shaped structure43 are formed by the use of plates of different thicknesses, i.e., bythe use of the thin plates 28 and 29 and the thick plates 30 and 31which differ from each other in thickness, to provide the arm 21 whichis reduced in weight and satisfactory in rigidity as an operating arm.

Especially, according to the present embodiment, the boss mounting thickplates 41 and 42 are welded to longitudinal ends of the wide plate-likematerial 27 which is composed of the thin plates 28 and 29 and the thickplates 30 and 31, and then the boss mounting thick plate 41 is bent intoU-shape together with the wide plate-like material 27 to form theU-shaped structure 43.

Therefore, the boss mounting thick plate 41, to be formed into the bossmount member 23, can be bent together with the wide plate-like material27 for reducing the number of steps and enhancing the efficiency of thearm fabrication process.

Besides, the boss mounting thick plates 41 and 42 which are joined withlongitudinal ends of the wide plate-like material 27 serve to suppressadverse effects of loads in bending operation such as tensile loads andcompression loads on the thin plates 28 and 29 of the wide plate-likematerial 27. Namely, the boss mounting thick plates 41 and 42 can beused as reinforcing members for the thin plates 28 and 29. Furthermore,since the boss mounting thick plate 41 is substantially same inthickness as the thick corner plates 30, stress and loads are uniformlydistributed at the time of bending these plates together.

Now, turning to FIGS. 15 and 16, there is shown a third embodiment ofthe present invention. In following description of the third embodiment,those component parts which are identical with the counterparts in theforegoing first embodiment are simply designated by the same referencenumerals or characters to avoid repetitions of same descriptions.

In short, a feature of the third embodiment resides in that the opening32A of the U-shaped structure 32 is closed with a plate-like member 51.

In this instance, similarly to the plate-like member 33 in the foregoingfirst embodiment, the plate-like member 51 is composed of a centrallylocated thin plate 52 and right and left thick plates 53. In this case,however, the plate-like member 51 is formed in a greater width than theafore-mentioned plate-like member 33, and the upper surfaces of theright and left thick plates 53 are abutted against the lower surfaces ofthe U-shaped structure 32 (the thick plates 31) at joint portions 54.

At the joint portions 54, the thick plates 53 of the plate-like member51 are joined with the thick plates 31 of the lower side of the U-shapedstructure 32 securely with deep penetration by laser welding. Thus, theopening 32A of the U-shaped structure 32 is closed with the plate-likemember 51 to form a square tubular structure 22′ of a square shape incross section similarly to the square tubular structure 22 in theforegoing first embodiment.

Being arranged as described above, the present embodiment can producesubstantially the same effects as the foregoing first embodiment.Particularly in this case, the corner portions 22A′ on the upper side ofthe square tubular structure 22′ can be formed of the thick cornerplates 30, and an upper flat section 22B′ can be formed of the flat thinplate 28.

Further, the corner portions 22C′ on the lower side of the squaretubular structure 22′ can be formed in the vicinity of the jointportions 54 between the thick plates 31 and 53, and the lower flatsection 22D′ can be defined by the lower surface of the plate-likemember 51 (the thin plate 52). On the other hand, the flat sections 22E′at the right and left lateral sides of the square tubular structure 22′can be formed by the thin plates 29 between the thick plates 30 and 31.

Now, turning to FIG. 17, there is shown a fourth embodiment of thepresent invention. In the following description of the fourthembodiment, those component parts which are identical with counterpartsin the foregoing first embodiment are simply designated by the samereference numerals or characters to avoid repetitions of samedescriptions.

In short, a feature of the present embodiment resides in that a squaretubular structure 61, which is a major part of the arm 21, isconstituted by a U-shaped structure 65 which is composed of flat thinplates 62 and 63 and thick corner plates 64, and a plate-like member 66which is joined to close an opening on the lower side of the U-shapedstructure 65.

In this instance, similarly to the wide plate-like material 27 in theforegoing first embodiment, the flat thin plates 62 and 63 and the thickcorner plates 64 are joined side to side by butt welding, and formedinto the U-shaped structure 65 by bending at the positions of the thickcorner plates 64 on a press.

The plate-like member 66 consists of a single steel plate which is sameas the thick corner plate 64 in thickness and larger in width than theafore-mentioned plate-like member 33. An upper surfaces of the right andleft side portions of the plate-like member 66 are abutted against thelower side of the U-shaped structure 65 (of the thin plates 63) andjoined with the latter at joint portions 67.

The joint portions 67 are formed by welding opposite side portions ofthe plate-like member 66 to lower surfaces of the thin plates 63 of theU-shaped structure 65 by laser welding to form strong joint portionswith deep penetration. As a consequence, the opening on the lower sideof the U-shaped structure 65 is closed with the plate-like member 66 toform a square tubular structure 61 of a square shape in cross sectionsimilarly to the square tubular structure 22 in the first embodimentdescribed above.

Being arranged in the manner as described above, the present embodimentcan produce substantially the same effects as in the foregoing firstembodiment. Particularly in this case, corner portions 61A on the upperside of the square tubular structure 61 are formed by the thick cornerplates 64, and an upper flat section 61B is formed by the flat thinplate 62.

Further, corner portions 61C on the lower side of the square tubularstructure 61 are formed in the vicinity of joint portions 67 between athin plate 63 and the plate-like member 66, and a lower flat section 61Dis defined by a lower surface of the plate-like member 66. On the otherhand, flat sections 61E at the right and left lateral sides of thesquare tubular structure 61 are formed by the flat thin plates 63.

Now, turning to FIGS. 18 through 21, there is shown a fifth embodimentof the present invention. In the following description of the fifthembodiment, those component parts which are identical with counterpartsin the foregoing first embodiment are simply designated by the samereference numerals or characters to avoid repetitions of sameexplanations.

In short, a feature of the present embodiment resides in that a squaretubular structure 71, which is a major part of the arm 21, is formed bya U-shaped structure 74 which is composed of a thick corner plate 72 andright and left flat thin plates 73, and a plate-like member 75 which isjoined in such a way as to close an opening on the lower side of theU-shaped structure 74, as shown in FIGS. 21 and 22.

In this instance, a wide plate-like material 74′, which is a startingmaterial to be formed into the U-shaped structure 74, is preparedsubstantially in the same manner as the wide plate-like material 27 inthe first embodiment, namely, by welding thick corner plate 72 and flatthin plates 73 side to side as shown in FIGS. 18 and 19. Particularly inthis case, the wide plate-like material 74′ is formed into the U-shapedstructure 74 on a press as shown in FIG. 20 by bending the thick cornerplate 72 along folding lines 72A indicated by broken lines in FIG. 18.

Further, similarly to the plate-like member 33 in the first embodiment,the plate-like member 75 is composed of a centrally located thin plate76 and right and left thick plates 77. However, in this case, theplate-like member 75 is formed in a larger width than the plate-likemember 33, and the upper surfaces of the right and left thick plates 77are abutted against the lower side of the U-shaped structure 74 (thethin plates 73) and joined with the latter at joint portions 78.

At the joint portions 78, the thick plates 77 of the plate-like member75 are welded to the thin plates 73 on the lower side of the U-shapedstructure 74 securely by laser welding with deep penetration. As aresult, the opening on the lower side of the U-shaped structure 74 isclosed with the plate-like member 75 to form a square tubular structure71 of a square shape in cross section similarly to the square tubularstructure 22 in the foregoing first embodiment.

Being arranged in the manner as described above, the present embodimentcan produce substantially the same effects as the first embodiment.Particularly in this case, the corner portions 71A on the upper side ofthe square tubular structure 71 are formed by right and left portions ofthe thick corner plate 72, and an upper flat section 71B is formed by atransversely intermediate portion of the thick corner plate 72.

Further, corner portions 71C on the lower side of the square tubularstructure 71 are formed in the vicinity of the joint portions 78 betweena thin plate 73 and the plate-like member 75 (one of the thick plates77), and a flat section 71D on the lower side is defined by a lowersurface of the plate-like member 75 (the thin plate 76). On the otherhand, flat sections 71E at the right and left lateral sides of thesquare tubular structure 71 are formed by the thin plate 73 as a flatthin plates.

Now, turning to FIG. 22, there is shown a sixth embodiment of thepresent invention. In the following description of the sixth embodiment,those component parts which are identical with counterparts in theforegoing first embodiment are simply designated by the same referencenumerals or characters to avoid repetitions of same explanations.

In short, a feature of this embodiment resides in that a square tubularstructure 81, which forms a major part of the arm 21, is constituted bya U-shaped structure 85 which is composed of thick corner plate 82,thick plates 83 and right and left flat thin plates 84, and a plate-likemember 86 which is assembled in such a way as to close an opening on thelower side of the U-shaped structure 85.

In this instance, similarly to the wide plate-like material 74′ of theforegoing fifth embodiment shown in FIGS. 18 and 19, the thick cornerplate 82, the thick plates 83 and flat thin plates 84 are welded side toside beforehand, and then formed into U-shape by bending right and leftportions of the thick corner plate 82 on a press to obtain a U-shapedstructure 85.

Similarly to the plate-like member 33 in the first embodiment, theplate-like member 86 is composed of a centrally located thin plate 87and right and left thick plates 88. Particularly in this case, however,the plate-like member 86 is formed in a larger width than the plate-likemember 33, and the upper surfaces of the right and left thick plates 88are abutted against the lower side of the U-shaped structure 85 (thethick plates 83) and joined with the latter at joint portions 89.

At the joint portions 89, the thick plates 88 of the plate-like member86 are welded to the thick plates 83 on the lower side of the U-shapedstructure 85 securely by deep penetration laser welding. As a result,the opening on the lower side of the U-shaped structure 85 closed by theplate-like member 86 to form a square tubular structure 81 of a squareshape in cross section similarly to the square tubular structure 22 inthe above-described first embodiment.

Being arranged in the manner as described above, the present embodimentcan produce substantially the same effects as the foregoing firstembodiment of the invention. Particularly in this case, the cornerportions 81A on the upper side of the square tubular structure 81 areformed by right and left side portions of the thick corner plate 82, andan upper flat section 81B is formed by a transversely intermediateportion of the thick corner plate 82.

Further, the corner portions 81C on the lower side of the square tubularstructure 81 are formed in the vicinity of the joint portions 89 betweena thick plate 83 and the plate-like member 86 (a thick plate 88), and alower flat section 81D is defined by a lower surface of the plate-likemember 86 (the thin plate 87). On the other hand, flat sections 81E atthe right and left lateral sides of the square tubular structure 81 areformed by the flat thin plates 84.

Turning now to FIG. 23, there is shown a seventh embodiment of thepresent invention. In the following description of the seventhembodiment, those component parts which are identical with counterpartsin the foregoing first embodiment are simply designated by the samereference numerals or characters to avoid repetitions of samedescriptions.

In short, a feature of the present embodiment resides in that a squaretubular structure 91, a major part of the arm 21, is constituted by aU-shaped structure 95 which is composed of flat thin plates 92 and 93and thick corner plates 94, and a plate-like member 96 which isassembled to close an opening on the upper side of the U-shapedstructure 95.

Namely, according to the present embodiment, the U-shaped structure 95is located on the lower side of the plate-like member 96. The U-shapedstructure 96 is formed in U-shape in cross section with an opening onthe upper side as shown in FIG. 23, and the plate-like member 95 issecurely assembled in such a way as to close the opening on the upperside of the U-shaped structure 95 at joint portions 97 as describedbelow.

In this instance, similarly to the wide plate-like material 27 in thefirst embodiment, the flat thin plates 92 and 93 and the thick cornerplates 94 are joined by side to side butt welding, and formed intoU-shape on a press by bending the thick corner plates 94 as the flatthin plate 93 is turned up to obtain a U-shaped structure 95.

The plate-like member 96 is formed by a single steel plate which issimilar to the thick corner plates 94 in thickness, and larger than theabove-described plate-like member 33 in width, and the left and rightportions of the plate-like member 96 is joined on its lower side withupper end faces of the U-shaped structure 95 (the flat thin plates 93)at joint portions 97.

More particularly, at joint portions 97, right and left side portions ofthe plate-like member 96 are joined with the thin plates 93 on the upperside of the U-shaped structure 95 securely by deep penetration laserwelding. As a result, an opening of the U-shaped structure 95 is closedby the plate-like member 96 to form a square tubular structure 91 whichis in a square shape in cross section similarly to the square tubularstructure 22 in the foregoing first embodiment.

Being arranged in the manner as described above, the present embodimentcan produce substantially the same effects as the above-described firstembodiment. In this instance, the corner portions 91A on the upper sideof the square tubular structure 91 are formed in the vicinity of thejoint portions 97 between a flat thin plate 93 and the plate-like member96. An upper flat section 91B is defined by an upper surface of theplate-like member 96.

Further, corner portions 91C on the lower side of the square tubularstructure 91 are formed by the thick corner plates 94, and a lower flatsection 91D is formed by the flat thin plates 92. On the other hand,flat sections 91E at the right and left lateral sides of the squaretubular structure 91 are formed by the flat thin plates 93.

Turning now to FIGS. 24 through 28, there is shown an eighth embodimentof the present invention. In the following description of the eighthembodiment, those component parts which are identical with counterpartsin the foregoing first embodiment are simply designated by the samereference numerals or characters to avoid repetitions of sameexplanations.

In short, a feature of the present embodiment resides in that flat thinplates and thick corner plates are joined by side to side butt weldingsuch that surfaces of the respective plates are disposed substantiallyflush with each other on one side but indented on the other side at thepositions of the thin and thick plates.

In the drawings, indicated at 101 is a square tubular structure which isadopted by the present embodiment. This square tubular structure 101 isformed substantially in the same manner as the square tubular structure22 in the first embodiment. In this instance, as shown in FIG. 28, thesquare tubular structure 101 is constituted by right and left uppercorner portions 101A, an upper flat section 101B which is formed betweenthe right and left upper corner portions 101A, right and left lowercorner portions 101C, a lower flat section 101D which is formed betweenthe lower corner portions 101C, and right and left side flat sections101E which are formed between upper and lower corner portions 101A and101C.

Indicated at 102 is a wide plate-like material to be formed into asquare tubular structure 101. This wide plate-like material 102 isformed substantially in the same manner as the wide plate-like material27 in the above-described first embodiment. In this instance, as shownin FIGS. 24 and 25, the wide plate-like material 102 is formed byalternately joining one of flat thin plates 103 and 104 and one of thickcorner plates 105 and thick plates 106 by side to side butt welding. Inthis regard, the respective plates are butt joined by high energydensity welding like laser welding which can ensure deep penetration.

At the time of welding together the respective plate materials forpreparation of the wide plate-like material 102, the flat thin plates103 and 104 are disposed substantially flush with the thick cornerplates 105 and thick plates 106 on one side (on the lower side), butsurfaces of the thin and thick plates are indented on the other side (onthe upper side).

Indicated at 107 is a U-shaped structure which is formed by bending thewide plate-like material 102. This U-shaped structure 107 is formed bybending the thick corner plates 105 of the wide plate-like material 102into a convexly curved shape along folding lines 105A indicated bybroken lines in FIG. 24, and is formed into U-shape in cross sectionthrough plastic deformation as shown in FIGS. 26 and 27.

As a bending operation proceeds, the right and left thick corner plates105 are bent into L-shape in cross section as shown in FIG. 26 to makecorner portions 101A of a square tubular structure 101 shown in FIG. 28.The centrally positioned flat thin plate 103 defines an upper flatsection 101B of the square tubular structure 101.

Further, the right and left flat thin plates 104 define flat sections101E at the right and left lateral sides of the square tubular structure101. As shown in FIG. 26, an opening 107A is formed on the lower side ofthe U-shaped structure 107 between the right and left thick plates 106.The opening 107A is closed by a plate-like member 108, which will bedescribed below.

Outer surfaces of the U-shaped structure 107 contain raised and indentedsurfaces 107B and 107C which are attributable to the difference inthichness between the flat thin plates 103 and 104 and the thick cornerplates 105. The inner surface of the U-shaped structure 107 is joinedflush with surfaces.

Indicated at 108 is a plate-like member which makes up the squaretubular structure 101 together with the U-shaped structure 107. As shownin FIG. 26, the plate-like member 108 is composed of a centrallypositioned thin plate 109 and right and left thick plates 110 which arejoined side to side with the thin plate 109 by high energy welding orthe like.

In this instance, as shown in FIG. 26, the thin plate 109 in the centerposition is joined with the right and left thick plates 110 by side toside butt welding such that its surface is disposed substantially flushwith the right and left thick plates on the upper side but indented onthe lower side of the plate-like member 108.

The plate-like member 108 is abutted against the opening 107A of theU-shaped structure 107 (against lower ends of the thick plates 106) andsecurely joined with the thick plates 106 at joint portions 111 by highenergy density welding as shown in FIG. 28.

As a result, the opening 107A of the U-shaped structure 107 is closedwith the plate-like member 108 to form a square tubular structure 101which is of a square shape in cross section as shown in FIG. 28. Lowerright and left corner portions 101C of the square tubular structure 101are formed in the vicinity of the joint portions 111 between a thickplate 106 of the U-shaped structure 107 and a thick plate 110 of theplate-like member 108, and a flat section 101D on the lower side of thesquare tubular structure 101 is defined by a lower surface of theplate-like member 108.

Being arranged in the manner as described above, the present embodimentcan produce substantially the same effects as the foregoing firstembodiment. Particularly in the case of the present embodiment, at thetime of preparing the wide plate-like material 102 by side to side buttwelding, the flat thin plates 103 and 104 are disposed substantiallyflush with the thick corner plates 105 and thick plates 106 on the lowerside but indented on the upper side of the wide plate-like material 102as seen in FIGS. 24 and 25.

Therefore, in this case it becomes possible to prevent exertion oftensile loads at the joint portions 112 between the flat thin plate 103and the thick corner plates 105 at the time of bending the thick cornerplates 105 of the wide plate-like material 102 into a convexly curvedshape on a press to form a U-shaped structure 107 as shown in FIG. 27,thus preventing development of cracks between the joint portions 112.

Namely, when the thick corner plates 105 are bent on a press, there is atendency that tensile loads are exerted on the outer side of theU-shaped structure 107 (on the side of the raised and indented surface107B) as indicated by arrows A in FIG. 27, and compressive loads areexerted on the inner side of the U-shaped structure 107 as indicated byarrows B. In this case, however, the flat thin plate 103 is joinedsubstantially flush with the thick corner plates 105 on the inner sideand indented from a raised and indented surface 107B on the outer side.

As a consequence, the joint portions 112 between the flat thin plate 103and the thick corner plates 105 are almost free from actions of tensileloads in the direction of arrows A, and free from degradations instrength which would otherwise be caused under the influence of tensileloads. Conversely, compressive loads in the direction of arrows B areexerted on the joint portions 112 between the flat thin plate 103 andthe thick corner plates 105. However, no adverse effects are imposed onthe joint portions 112 by the compressive loads. Namely, in contrast tothe tensile loads in the direction of arrows A which tend to pull apartthe joint portions 112, the compressive loads in the direction of arrowsB as shown FIG. 27 do not act to impose any adverse effects on the jointportions 112.

It follows that the joint portions 112 are prevented from rupturing andallowed to retain sufficient strength. As compared with the U-shapedstructure 32 in the foregoing first embodiment, the joint portions 112of the U-shaped structure 107 are more reliably reduced in residualtensile stress and are markedly improved in resistance to cracking andfatigue life.

Further, in this case, the raised and indented surfaces 107B and 107C,which are attributable to the difference in thickness between the flatthin plate 103 or 104 and the thick corner plate 105, are exposed on theouter side of the square tubular structure 101 to form the U-shapedstructure 107. These raised and indented surfaces 107B and 107C can beused to give certain design effects to the outer side of the squaretubular structure 101, for example, sturdiness in design, for thepurpose of attaching an enhanced commercial value to the square tubularstructure 101 as an operating arm of a construction machine.

Turning now to FIGS. 29 and 30, there is shown a ninth embodiment of thepresent invention. In the following description of the ninth embodiment,those component parts which are identical with counterparts in theforegoing eighth embodiment are simply designated by the same referencenumerals or characters to avoid repetitions of same explanations.

In short, a feature of the present embodiment resides in that a wideplate-like material 122, to be formed into a square tubular structure121, is prepared by the use of thick corner plates 123 as shown in FIG.30. Side edges of the thick corner plates 123 are chamfered to providesloped surfaces 123A and 123B.

In this instance, the wide plate-like material 122 is formed byalternately joining one of flat thin plates 103 and 104 and one of thickcorner plates 123 and thick plates 124 by side to side butt weldingsubstantially in the same manner as the wide plate-like material 102 inthe foregoing eighth embodiment.

Particularly in the case of the present embodiment, however, the wideplate-like material 122 differs from the counterpart in the precedingembodiment in that the thick corner plates 123 are each provided withsloped surfaces 123A and 123B at opposite side edges. In addition, inthis case, each one of the thick plates 124 of the wide plate-likematerial 122 is provided with a sloped surface 124A.

In this case, the wide plate-like material 122 is also formed by bendingthe thick corner plates 123 on a press to fabricate a U-shaped structure125 which is U-shape in cross section. An opening 125A on the lower sideof the U-shaped structure 125 is closed by a plate-like member 126 asshown in FIG. 29.

In this instance, similarly to the plate-like member 108 in theforegoing eighth embodiment, the plate-like member 126 is composed of acentrally located thin plate 109 and right and left thick plates 127. Inthis particular case, however, each one of the thick plates 127 of theplate-like member 126 is provided with a chamfered or sloped surface127A at a side edge.

Outer surfaces of the square tubular structure 121 (the U-shapedstructure 125) contain raised and indented surfaces 125B and 125C whichare attributable to the difference in thickness between the flat thinplates 103 and 104 and the thick courner plates 123. The inner side ofthe square tubular structure 121 are joined flush with surfaces.

Being arranged in the manner as described above, the present embodimentcan produce substantially the same effects as the foregoing eighthembodiment. Particularly in this case, the corner portions 121A on theupper side of the square tubular structure 121 are formed by the thickcorner plates 123, and an upper flat section 121B is formed by the flatthin plate 103.

Further, the corner portions 121C on the lower side of the squaretubular structure 121 are formed in the vicinity of the joint portions111 between the thick plates 124 and the plate-like member 126, and alower flat section 121D is defined by the lower side of the plate-likemember 126. The flat sections 121E at the right and left lateral sidesof the square tubular structure 121 are formed by the flat thin plates104.

In this particular embodiment, however, right and left side edges ofeach one of the thick corner plates 123 are chamfered into the slopedsurfaces 123A and 123B, and each one of the thick plates 124 and 127 isprovided with a sloped surface 124A or 127A. Edges of raised andindented surfaces 125B and 125C which are exposed on the outer side ofthe square tubular structure 121 (the U-shaped structure 125) aresmoothened by the sloped surfaces 123A, 123B, 124A and 127A to add to acommercial value as an operating arm of a construction machine.

Now, turning to FIGS. 31 to 34, there is shown a tenth embodiment of thepresent invention. In the following description of the tenth embodiment,those component parts which are identical with counterparts in theforegoing first embodiment are simply designated by the same numeral orcharacters to avoid repetitions of same explanations.

In short, a feature of the present embodiment resides in that flat thinplates and thick corner plates are alternately joined by side to sidebutt welding in such a way as to form raised and indented surfaces onboth sides in a direction of thickness of a wide plate-like material.

In the drawings, indicated at 131 is a square tubular structure which isadopted by the present embodiment. This square tubular structure 131 isformed substantially in the same manner as the square tubular structure22 in the foregoing first embodiment, and, as shown in FIG. 31,constituted by right and left upper corner portions 131A, an upper flatsection 131B which is located between the each upper corner portions131A, right and left lower corner portions 131C, a lower flat section131D which is located between the each lower corner portions 131C, andright and left flat sections 131E which are located between an uppercorner portion 131A and a lower corner portion 131C.

Indicated at 132 is a wide plate-like material to be formed into asquare tubular structure 131. The wide plate-like material 132 is formedsubstantially in the same manner as the wide plate-like material 27 inthe first embodiment. In this instance, as shown in FIG. 32, the wideplate-like material 132 is formed by alternately joining one of flatthin plates 133 and 134 and one of thick corner plates 135 and thickplates 136 by side to side butt welding. More specifically, therespective plates are joined with each other by high energy densitywelding, for example, by deep penetration laser welding.

Particularly in this case, the flat thin plates 133 and 134, the thickcorner plates 135 and the thick plates 136 are aligned and joined atintermediate positions in the direction of thickness as to form raisedand indented surfaces on both sides in the direction of thickness (theupper and lower sides) for the wide plate-like material 132, as shown inFIG. 32.

Denoted at 137 is a jig table for use in preparing the wide plate-likematerial 132. As shown in FIG. 32, the upper surface of the jig table137 is provided with heightened surfaces 137A and 137B at positionscorresponding to the positions of the flat thin plates 133 and 134, andalternately provided with a sunken surface 137C and 137D betweenheightened surfaces 137A and 137B.

Prior to welding operation, flat thin plates 133 and 134 are placed onthe heightened surfaces 137A and 137B of the jig table 137,respectively, and thick corner plates 135 and thick plates 136 areplaced on the sunken surfaces 137C and 137D, respectively. At this time,as shown in FIG. 32, the flat thin plates 133 and 134 are set atpositions which are lower than those of the thick corner plates 135 andthe thick plates 136 by a dimension t. In this instance, for example,the dimension t is preferably set approximately at a value of ½ (t=T/2)where T stands for the thickness of the thick corner plates 135 andthick plates 136.

Indicated at 138 is a U-shaped structure which is formed by bending thewide plate-like material 132. This U-shaped structure 138 is formed bybending the thick corner plates 135 of the wide plate-like material 132into a convexly curved shape on a press, fabricating through plasticdeformation which is U-shape in cross section as shown in FIGS. 33 and34.

At this time, as a bending operation proceeds, the right and left thickcorner plates 135 are bent into L-shape in cross section as shown inFIG. 34 to serve as corner portions 131A of a square tubular structure131 which is shown in FIG. 31. The flat thin plate 133 in a centerposition becomes an upper flat section 131B on the upper side of thesquare tubular structure 131.

Further, the right and left flat thin plates 134 become flat sections131E at the right and left lateral sides of the square tubular structure131. As shown in FIG. 33, an opening 138A is formed on the lower side ofthe U-shaped structure 138 between the right and left thick plates 136,and the opening 138A is closed by a plate-like member 139, which will bedescribed hereinafter.

Formed on the outer side of the U-shaped structure 138 are raised andindented surfaces 138B and 138C due to a difference in thickness of theflat thin plates 133 and 134 from the thick corner plates 135. In thiscase, similar raised and indented surfaces are formed also on the innerside of the U-shaped structure 138.

Indicated at 139 is a plate-like member which constitutes the squaretubular structure 131 together with the U-shaped structure 138. As shownin FIG. 33, this plate-like member 139 is constituted by a centrallylocated thin plate 140, and right and left thick plates 141 which arejoined with opposite sides of the thin plate 140 by high energy densitywelding or the like.

Particularly in this case, as shown in FIG. 33, the thin plate 140 inthe center and the right and left thick plates 141 joined by side toside butt welding at such intermediate positions in the direction ofthickness as to form raised and indented surfaces both on upper andlower sides of the plate-like member 139. The plate-like member 139 isabutted against the opening 138A of the U-shaped structure 138 (againstthe lower ends of the thick plates 136) and securely joined with thethick plates 136 by high energy density welding or the like at jointportions 142 as shown in FIG. 31.

As a consequence, the opening 138A of the U-shaped structure 138 isclosed by the plate-like member 139 from beneath to form a squaretubular structure 131 which is square in cross section as shown in FIG.31. The right and left corner portions 131C on the lower side of thesquare tubular structure 131 are formed in the vicinity of the jointportions 142 between the thick plates 136 of the U-shaped structure 138and the thick plates 141 of the plate-like member 139, and a flatsection 131D on the lower side of the square tubular structure 131 isdefined by a lower surface of the plate-like member 139.

Being arranged in the manner as described above, the present embodimentcan produce substantially the same effects as the foregoing firstembodiment. However, according to the present embodiment, at the time ofpreparing the wide plate-like material 132, the flat thin plates 133 and134 are abutted and joined with the thick corner plates 135 and thethick plates 136 at such intermediate positions as to form raised andindented surfaces on both sides of the wide plate-like material as shownin FIG. 32.

Therefore, it becomes possible to suppress exertion of tensile loads onthe welded joints 143 between the flat thin plate 133 and the thickcorner plates 135 when the thick corner plates 135 of the wideplate-like material 132 are bent into a convexly curved shape to form aU-shaped structure 138 as shown in FIG. 33, for example, for the purposeof preventing cracks from developing at the welded joints 143 startingfrom an end of a welding bead.

Especially in a case where the dimention t, corresponding to a dimensionof indentation of the flat thin plate 133 from raised surfaces of thethick corner plates 135 is set, for example, approximately at a value of½ (t=T/2) T where stands for the thickness of the thick corner plates135 as shown in FIG. 34, it becomes possible to suppress and minimizeexertion of tensile stresses acting in the directions of arrows A at thewelded joints 143 between the flat thin plate 133 and the thick cornerplates 135.

Thus, the welded joints 143 between the flat thin plate 133 and thethick corner plates 135 are almost free from tensile stresses acting inthe directions of arrows A. Namely, there is little possibility of thewelded joints 143 being degraded in strength under the influence oftensile stresses. With regard to compressive stresses exerted on thewelded joints 143 acting in the directions of arrows B, thesecompressive stresses pose no adverse effects on the welded joints 143between the flat thin plate 133 and the thick corner plates 135.

As a consequence, the welded joints 143 are unsusceptible to rupturingfrom an end of a welding bead, and thus sufficient strength isguaranteed for each one of the welded joints 143. Besides, the weldedjoints 143 of the U-shaped structure 138 are markedly reduced inresidual tensile stress and can prolong fatigue life to a considerabledegree, as compared with the U-shaped structure 32 in the foregoingfirst embodiment.

Further, similarly to the foregoing eighth embodiment, in the squaretubular structure 131 which is constituted by the U-shaped structure 138according to the present embodiment, the raised and indented surfaces138B and 138C can add a sort of design effects to the outside of thesquare tubular structure 131 for the purpose of enhancing a commercialvalue as an operating arm of a construction machine.

In the foregoing embodiments of the invention, by way of example thesquare tubular structures 22, 22′, 61, 71, 81, 91, 101, 121 and 131 areapplied as an operating arm like the arm 21 on an offset boom typeworking mechanism 11. However, the present invention is not limited tothe particular examples shown, and, for example, the square tubularstructures can be similarly applied to the lower boom 12 and upper boom13 shown in FIG. 1.

Further, the present invention can find application not only as anoperating arm on a working mechanism like the offset boom type workingmechanism 11, but also as an operating arm of a working mechanism 161 ona hydraulic excavator 151 which is shown in a modification of FIG. 35 asa standard machine. In this case, as a construction machine, thehydraulic excavator 151 is largely constituted by a crawler typeautomotive base structure 152, a revolving structure 153 and a workingmechanism 161.

The revolving structure 153 includes a revolving frame 154, a cab 155providing an operating room to be occupied by an operator at the controlof the machine, a housing cover 156 serving as an exterior cover, and acounterweight 157.

The working mechanism 161 is liftably provided on a front side of therevolving structure 153, including a boom 162, an arm 163 and a frontattachment like a bucket 164. A boom cylinder 165 is provided betweenthe revolving frame 154 and the boom 162, and an arm cylinder 166 isprovided between the boom 162 and the arm 163. Further, a bucketcylinder 169 for a front attachment is provided between the arm 163 andthe bucket 164 through links 167 and 168.

In this case, as an operating arm, a square tubular structure similar tothe square tubular structures 22, 22′, 61, 71, 81, 91, 101, 121 and 131in the above-described embodiments can be applied to the boom 162 and tothe arm 163 as well.

Further, the present invention is widely applicable not only to crawlertype hydraulic excavator but also to a working mechanism (front part) ofa wheel type hydraulic excavator, a dredger or other constructionmachine like a hydraulic crane or the like.

On the other hand, the square tubular structure 91 adopted in theseventh embodiment is basically same construction as the square tubularstructure 61 of the fourth embodiment shown in FIG. 17 except that thesquare tubular structure is turned upside down. In this regard,similarly to the square tubular structure 91, the square tubularstructures 22, 22′, 71, 81, 101, 121 and 131 in the foregoing first tothird embodiments, fifth embodiment, sixth embodiment and eighth totenth embodiments can be applied in an inverted form.

Furthermore, in the foregoing eighth to tenth embodiments of theinvention, raised and indented surfaces 107B and 107C (125B, 125C, 138Band 138C) are formed on the outer side of a square tubular structure 101(121 or 131) on purpose utilizing a difference in thickness betweenjoined plates. According to the present invention, if desired, raisedand indented surfaces as in the eighth to tenth embodiments may besimilarly formed on the outer side of the square tubular structures 22,22′, 61, 71, 81 and 91 in the first to seventh embodiments of theinvention.

1. A method of fabricating an operating arm for a construction machine,said operating arm being constituted by a plural number of joined platesand in the shape of a square tubular structure in cross section, saidmethod comprising the steps of: preparing a wide plate-like materialhaving alternately thick and thin wall portions in a transversedirection by butt welding side to side said plural number of joinedplates in different thicknesses; bending said wide plate-like materialalong said thick wall portions to form thick wall corner portions ofsaid square tubular structure, and to form a U-shaped structure having aU-shape in cross section with an opening on one side, through plasticdeformation; and welding a separate plate-like member to said U-shapedstructure to close said opening to form said square tubular structure.2. The method of fabricating an operating arm for a construction machineas defined in claim 1, wherein said preparing step further compriseswelding a boss mounting thick plate to be formed into a boss mountmember to one longitudinal end of said wide plate-like material, andbending said boss mounting thick plate into a U-shape in cross sectionsimultaneously with the bending of said wide plate-like material to formsaid U-shaped structure.
 3. The method of fabricating an operating armfor a construction machine as defined in claim 1, wherein said thin andthick plates are joined by high energy density welding of deeppenetration in said preparing step.
 4. The method of fabricating anoperating arm for a construction machine according to claim 1, whereinsaid preparing step includes positioning surfaces of said thin wallportion flush with surfaces of said thick wall portions on an outside inthe direction of thickness but indented from said thick wall portions onan inside in the direction of thickness prior to butt welding.
 5. Themethod of fabricating an operating arm for a construction machineaccording to claim 1, wherein said preparing step includes positioningsurfaces of said thin wall portions indented from surfaces of said thickwall portions on an outside in the direction of thickness but flush withsaid thick wall portions on an inside in the direction of thicknessprior to butt welding.
 6. The method of fabricating an operating arm fora construction machine according to claim 1, wherein said preparing stepincludes positioning surfaces of said thin wall portions indented fromsurfaces of said thick wall portions on both sides in the direction ofthickness prior to butt welding.